The invention relates to a laser scanner and a method for optically scanning and measuring an environment. More particularly, the invention relates to laser scanners used to scan and measure a variety of interior and exterior spaces. For instance, these may be manufacturing buildings in which specific facilities are planned. In this case, the laser scanner is placed on a stand into the space to be measured, and the measuring head is slowly rotated about a vertical axis, while a rotor—arranged in the measuring head—with a light transmitter rotates at a significantly higher rotational speed about a horizontal axis. In this way, the fast rotation results in a light fan being generated in a vertical plane, said light fan being slowly rotated through e.g. 360°, so that finally the entire environment has been scanned. The emitted light beams are reflected from the points in the environment, and the reflected light beam is received by the measuring head again. In this case, both the distance of the respectively measured point in the environment and the reflectivity of said point are determined, so that finally a faithful imaging with a solid angle of ideally 360° arises.
In another application of such scanners, elongated cavities, such as tunnels, are measured. In this case, the light transmitter rotates about a horizontal axis and it is moved along the tunnel to be measured. In this way, it is possible to monitor the state of tunnels, to determine the clearance at any point in the tunnel, etc.
Laser scanners of the abovementioned type are usually specified for a specific distance range. This means that objects can be detected and measured at greater or lesser distance depending on the class of the components used. Typically, the intensity of the reflected light beam depends on the distance of the measurement point and its reflectivity. The intensity of the reflected light beam results in a certain gray-scale value provided by the receiver.
In applications of laser scanners for a large distance range, where objects situated at a large distance are still to be reliably identified and measured, and in applications with a large gray-scale value range, where measurement points having a relatively poor reflectivity are to be reliably differentiated, the receiver's dynamic range limits are soon encountered. It must be taken into account that the intensity of the reflected light beam decreases more than proportionally with the distance. Thus, the quantity of light that is reflected from an object at a distance of 50 m amounts to only approximately 4×10−6 of the quantity of light that is reflected from an object at a distance of 0.1 m. The range of different gray-scale values is an additional factor. According to the current prior art there are no receivers available which cover such a wide dynamic range.
DE 22 16 765 C3 discloses a method and a device for distance measurement. This involves determining first of all the distance to a specific measurement point and optionally also the reflectivity of the measurement point. The distance is determined from a propagation time measurement between an emitted light pulse and the light pulse reflected from the measurement point. Errors can occur if the propagation time measurement is ended and the received signal, i.e. a signal pulse generated from the reflected light pulse, reaches a specific trigger level but this signal pulse has an undefined maximum amplitude. In order to preclude this error, a regulation is performed which brings about an adjustment in such a way that the signal pulse is raised from a lower initial value until it has reached a defined level. The regulation works by adjusting either the transmission power or the reception gain prior to the generation of the signal pulse. The reflectivity is determined by detecting the amplitude of the signal pulse before being raised to the defined level and comparing it with a predetermined transmission power. In this case, furthermore, from the measured distance, a weighting is additionally carried out in order to calculate out the dependence of the amplitude of the reflected light pulse on the distance of the measurement point.
The known device and the known method are thus limited for measurements at a single measurement point, because the measurement point has to be intrinsically optimized in each case by adjusting the transmission power and/or the reception gain. This precludes scanning 2D or 3D measurements of an environment.
Furthermore, the reflectivity of the measurement point can be determined only when the absolute transmission power in the system that has not yet been adjusted is known. The inclusion of the distance finally determined and the weighting of the reflection value with this distance lead, finally, to an absolute value of the reflectivity at the location of the measurement point, but not to the gray-scale value received at the location of the measuring apparatus. This is because said gray-scale value is independent of distance. Specifically, in the case where an environment is represented in the manner of a photograph, each point has, for an observer, a gray-scale value that says nothing about how far away the point is from the observer. Therefore, image recordings of an environment cannot be produced by means of the known procedure.